Motor control system with linear acceleration circuit

ABSTRACT

A DC motor control system is energized over power semiconductor switches controlled by firing circuits. A control amplifier is connected to regulate the firing circuits and thus regulate motor energization. An adjustable potentiometer provides a speed reference signal. A linear acceleration stage, coupled between the adjustable potentiometer and the control amplifier, translates step function increases of the potentiometer setting into a smooth speed increase signal. The linear acceleration stage includes a capacitor connected in a charging circuit to provide this smooth speed increase signal in response to turnoff of a transistor which, when conducting, disables the charging circuit. An operational amplifier is connected to turn off the transistor and allow the capacitor to charge when the speed control potentiometer is adjusted to signal a speed increase. Responsive to a speed decrease, the transistor is immediately gated on to discharge the capacitor and subsequently assist the system in preventing motor coast-down below the new speed setting.

[72] inventors ThomasPantelakis 3,l63,8l2 l2/l964 Greening 318/400Margate; Dudley D. Nye. Jr.. Fort Lauderdale, both E'mml'rler ons L R IHa Assistant Exammer-Thomas Langer [2H AppL 837,456 AriglrnysjlDonald W.Banner. William S. McCurry and John 221 Filed June 30, 1969 PatentedJuly 13, I971 '[73] Ass'gnee Bofg'wamercorpomfion ABSTRACT: A DC motorcontrol system is energized over chlcago' power semiconductor switchescontrolled by firing circuits. A control amplifier is connected toregulate the firing circuits {54] MOTOR CONTROL SYSTEM WITH LINEAR ind[ltlUS reguliate motor;j enirgization. Aln idi ustable poten-ACCELERATION CIRCUIT ome er prov! es a spec re erence signa mear acceeratron stage, coupled between the ad ustable potentiometer and 8Claims, 1 Drawing Fig.

the control amplifier, translates step function increases of the [52]U.S. Cl 318/391, potentiometer setting into a smooth speed increasesignal. The 318/332, 318/403 linear acceleration stage includes acapacitor connected in a 1 Int-Cl H lp 1/04 charging circuit to providethis smooth speed increase signal Field of Search v. 318/400. inresponse to turnoff of a transistor which, when conducting 391. 331. 345disables the charging circuit. An operational amplifier is connected toturn off the transistor and allow the capacitor to [56] References c'iedcharge when the speed control potentiometer is adjusted to UNITED STATESPATENTS signal a speed increase. Responsive to a speed decrease, the3,366,861 1/1968 Dudler H 318/345 transistor is immediately gated on todischarge the capacitor 2929.980 3/1960 Angerum 318 400 and subsequentlyassist the system in preventing motor coast- 3,037,157 5/1962 Young318/400 down below the new Speed setting 4@ POWER CIRCUIT 2i 23 W 28 44M 43$]Z 45 Firing 21 c t s lrcu 4 Z '26 Z 5 29 46 44 4s 48 F ring 45-47- 49-1- CIFCUI? 1 30 48 5 67 hi Firing I. Circuit 49 T t i, 98 I00 I63 I A, v:-

4' l T T L. -60 w 32 l on'irol 93 a 6 V i 9 a VJ I =Amplifier 3 i EIOI l:86 5 7 96 85 '0 i4 I 1 9s I06 ::8\ I r87 R8 i 94 l a as l Ll E AR ACCELERATION STAGE 6?.

MOTOR CONTROL SYSTEM WITH LINEAR ACCELERATION CIRCUIT BACKGROUND OF THEINVENTION Various types of speed control arrangements have been employedin the regulation of DC motors. Frequently a potentiometer or otheradjustable arrangement is utilized to provide a reference signal whichultimately determines the motor speed. However, when the potentiometeris suddenly adjusted in a direction to increase the motor speed, if thissudden adjustment is translated as a step function signal to the circuitwhich regulates the motor speed, this may cause damaging surge currentin the motor armature in a manner known to those familiar with controlsystems. In an effort to obviate such undesired system response variouscircuits have been developed and interposed between the speed referencepotentiometer and the circuit (such as a control amplifier) whichregulates the motor energization and thus determines its speed. Ingeneral these circuits attempt to provide a gradual, approximatelylinear change in the output control signal in response to receipt of astep function signal from the speed regulating potentiometer. In manyinstances such circuits have proved unsatisfactory, both because offailure to achieve sufficient linearity in the output signal, andbecause they cannot rapidly follow speed set-down signals, tending toprevent the motor from slowing down at the maximum rate to the new speedsetting.

It is therefore a primary consideration of this invention to provide alinear acceleration stage for a motor control system in which the outputsignal is virtually linear in response to an input increase-speed signalof the step function type.

Another salient consideration of this invention is to provide such alinear acceleration circuit which immediately follows reduced-speedsignals, and does not produce a ramp or gradually varying signal whenthe motor speed is reduced.

It is another important consideration to provide a system which utilizesa feedback amplifier to insure that the output of the linearacceleration stage is accurately regulated in the steady state to matchthe setting of the reference or speed" potentiometer.

SUMMARY OF THE INVENTION This invention is useful in various controlarrangements and finds particular utility in an energizing system for anelectrical motor in which electrical energy is passed to the motorthrough a power circuit including at least one semiconductor powerswitch. The system includes a control amplifier having an input circuit,and having an output circuit connected to regulate operation of thesemiconductor power switch and thus regulate energization of said motor.An adjustable reference means, such as a potentiometer, is connected toprovide a speed reference signal which varies as the reference means isadjusted.

Particularly in accordance with the present invention, a linearacceleration stage is coupled between the adjustable reference means andthe control amplifier input circuit, The linear acceleration stageincludes an operational amplifier connected to provide a switchingsignal as the adjustable reference means is varied to signal a speedincrease. A capacitor is connected in a charging circuit to charge andthus provide a smooth speed change signal for application to the controlamplifier input circuit. A semiconductor switch is connected to disablethe charging circuit until the switching signal is provided by theoperational amplifier, and also to rapidly discharge the capacitor whenthe adjustable reference means is varied to signal a speed decrease.

THE DRAWING The single FIGURE of the drawing, partly schematic andpartly in block form, shows the present invention incorporated in a DCmotor energizing system.

2 DETAILED DESCRIPTION OF THE INVENTION The drawing depicts a motorenergizing system in which the armature circuit of a motor 20 receivesenergy from a power circuit 21 which, in turn, is supplied with ACenergy over three input conductors 22, 23 and 24. The level of energypassed to the motor is determined by the conduction times ofsemiconductor power switches or silicon controlled rectifiers (SCRs) 25,26 and 27 which, in turn, are regulated by gating signals provided byfiring circuits 28, 29 and 30. The firing'circuits are regulated by anoutput signal received over circuit 31 from a control amplifier 32,which operates in response to different input signals.

Motor 20 is energized as DC energy is passed through power circuit 21and applied between conductors 33 and 34. When switches 35 and 36 areclosed, current flows through motor 20 in a first direction and effectsmotor rotation in a given angular direction. When switches 35, 36 areopened and switches 37, 38 are closed (by control components which arenot illustrated because they are well known), current flows throughmotor 20 in the opposite direction and effects motor rotation in theopposite angular direction.

Motor field winding 40 is coupled between conductors 22 and 33. Threediodes 41, 42 and 43 are respectively coupled in series with thesemiconductor switches 25, 26 and 27. The input conductors 2224 arerespectively connected to the common connections between each diode-SCRpair. Although the semiconductor switches 25-27 are illustrated assilicon controlled rectifiers, other components such as thyratrons,ignitrons, power transistors, transistors, electrondischarge devices andsimilar switching units can be used in their place. Only onesemiconductor power switch is required to regulate the level of motorenergization. Such switch can be coupled in a series circuit connection,in a chopper or DC- to-DC converter arrangement, in lieu of theillustrated threephase rectifier arrangement. Silicon controlledrectifier 25 receives gating signals over conductors 44, 45 from firingcircuit 28; SCR 26 receives gating signals over conductors 46 and 47from firing circuit 29; and semiconductor switch 27 receives firingsignals over conductors 48, 49 from firing circuit 30.

A freewheeling diode 50 is coupled between conductor 33 and ground.Because of the inductive reactance of the motor, the turnoff of a givensilicon controlled rectifier may terminate current supply while themotor tends to keep current flowing; the freewheeling diode 50 maintainsa path for the continuing current flow. A resistor 51 is coupled betweenconductor 34 and ground. A potentiometer 52 is coupled between conductor34 and ground, and the movable arm is coupled over a resistor 53 and aconductor 54 to a control amplifier 32. A pair of resistors 57, 58 arecoupled in series between conductor 33 and ground, and conductor 60 iscoupled between the midpoint of these two resistors and a first inputconnection of control amplifier 32.

In accordance with this invention, control amplifier 32 also receives aspeed reference signal over line 61 from a linear acceleration stage 62which in turn receives an input signal from the movable arm of anadjustable reference means, shown as a potentiometer 63. Linearacceleration stage 62 is utilized to translate a step function change ofthe setting of potentiometer 63 into a gradual, smooth transition of aspeed control signal applied over line 61 to the control amplifier stage32, and to preclude overshoot when the speed is set down.

Three diodes 64, 65 and 66 are coupled in series between conductor 54and ground to protect against an unduly large signal being passed overconductor 54 to the control amplifier stage if resistor 51 were open, orif for some other reason an excess of current were to be suddenlysupplied over resistor 53. A resistor 67 is coupled between ground and acommon conductor 68 coupled to the cathodes of all the semiconductorswitches 25, 26 and 27. Conductor 70 is coupled to the common connectionof resistor 67 and common conductor 68, and is also coupled to controlstage 32.

In the circuit of linear acceleration stage 62 an operational amplifier(op amp) 80 is provided to produce a switching signal as the adjustablereference means or potentiometer 63 is varied to signal a speedincrease. A capacitor 81 is connected in a charging circuit which alsocomprises a resistor 82 and a linear accelerator timing controlpotentiometer 83. Charging of capacitor 81 provides the requisite linearspeed change signal for-application over conductor 61 to the controlamplifier stage, when a step change signal is created by displacing themovable arm of potentiometer 63. A first semiconductor switch, shown asa PNP-ytpe transistor 84, is connected to disable the charging circuituntil the switching signal is provided by op amp 80. in addition, astransistor 84 is gated on responsive to an indication either that thenew (higher) speed has been reached, or that a speed reduction signal isprovided by set-down of potentiometer 63, transistor 84 conducts toeffect substantially instantaneous discharge of capacitor 81 and preventspeed overshoot or undershoot in the system.

In more detail, the terminals of op amp 80 are referenced internallyexcept for output terminal 7, to assist those skilled in the art topractice the invention with a minimum of experimentation. Input terminal10 is coupled through a series circuit comprising series-coupledresistors 85 and 86 to the movable arm of potentiometer 63. A capacitor87 is coupled between reference or ground conductor 88 and the commonconnection between resistors 85 and 86. The other input connection 12 iscoupled over a resistor 90 to conductor 61. Terminal 8 is connected to aterminal 91, for receiving a negative energizing potential, and terminal92 is connected to op amp terminal 3 for providing a positive voltagewhen the system is energized. Frequency compensation components includecapacitor 93 coupled between terminals and 7, and a series circuitincluding capacitor 94 and resistor 95, coupled between terminals 1 and14.

A resistor 96 is coupled in series between output terminal 7 of the opamp and the base of first semiconductor switch 84. This switch, shown asa PNP-type transistor, has its emitter coupled to ground and itscollector coupled to the common connection between capacitor 81,resistor 82, and the base of a second semiconductor switch shown asanother PNP-type transistor 97. Terminal 98 provides an input connectionfor receiving a negative energizing voltage and terminal 100 is asimilar connection for receiving a positive voltage, when the system isenergized. A series of circuit including a resistor 101 and a diode 102is coupled between output conductor 61 and reference conductor 88.Another resistor 103 is coupled between terminal 100 and the commonconnection between resistor 101 and diode 102.

Initially with no variation in the reference signal provided bypotentiometer 63, there is a negative-polarity signal at output terminal7 of op amp 80, and this signal is applied over resistor 96 to the baseof first transistor 84, maintaining this PNP-type transistor conducting.In effect the first semiconductor switch 84 disables the chargingcircuit 8183 until a step-up signal is provided from potentiometer 63.

Assuming now that the movable arm of potentiometer 63 is displacedupwardly, toward the terminal 105, this action provides a step functionsignal which is applied over resistors 86 and 85 to input connection ofthe op amp. This input signal produces a change in the polarity of theoutput signal at terminal 7, which is effective to turn off transistor84. Accordingly capacitor 81 begins to charge as current flows fromconductor 88, through capacitor 81, resistor 82, and potentiometer 83 toterminal 98. This charging action produces a ramp waveform signal whichis passed through second transistor 97 and over output conductor 61 tothe control amplifier stage.

Transistor 97 is connected in an emitter-follower configurationincluding emitter resistor 101. Diode 102, energized via resistor 103from a positive power supply source represented by terminal 100,provides approximately 0.7 volt bias to bring transistor 97 just to theconductive state when the reference voltage from potentiometer 63 andthe voltage across capacitor 81 is at substantially zero. This preventsany noticeable dead time in the ramp function output when setting thespeed up from zero.

As noted previously it is important to provide a substantially linearramp, or slope, of the signal passed over conductor 61. The rampwaveform of voltage rise on conductor 61 will build up until it matchesthe reference voltage set on input terminal 10 of op amp 80. This occursas a result of negative feedback through resistor to plus terminal 12 ofthe op amp. To assist in achieving this linearity, the level of supplyvoltage applied to terminal 98 of the charging circuit is selected suchthat the entire amplitude of variation of the speed change signaldeveloped across capacitor 81 is much smaller, by at least an order ofmagnitude, than the level of the voltage applied to terminal 98 andtoward which capacitor 81 is charged. Order of magnitude," as usedherein and in the appended claims, refers to variations by powers of 10(except for the same order of magnitude) with respect to a referencevalue. By way of example, taking 5 as a reference value, other valuesfrom onefifth to 5 times the reference value (from 1 to 25) areconsidered to be within the same order of magnitude as the referencevalue 5. Values from 0.1 to 1.0 are considered to be an order ofmagnitude less, referred to the value 5; 0.01 to 0.1 is considered inthe second order of magnitude less; and so forth. This circuitarrangement insures that the entire ramp waveform is generated duringthe initial charge of capacitor 81, a period much less than one timeconstant, which those skilled in the art appreciate is substantially alinear change in the signal passed over output conductor 61 when a stepfunction speed increase signal is received at input terminal 10 of theop amp.

Solely to assist those skilled in the art to practice the invention witha minimum of experimentation, and in no sense by way of limitation, atable of circuit component identifications and values for the linearacceleration stage 62 is set out below. In the stage op amp 80 was a0.15. integrated circuit, type PA-238. A negative 16 volt potentialrelative to ground conductor 88 was applied to terminal 98, andcapacitor 81 was charged only in the range of0 to 1.0 volt to effect therequisite speed change signal output and balance the circuit. Thus it isapparent that the voltage applied to terminal 98 is larger, by at leastan order of magnitude, than the voltage developed across capacitor 81 torepresent the smooth speed change signal. A 6 volt potential was appliedto terminal 91, and a +6 volt potential was applied to terminals 92 and100. The identifications and values of the other components were:

2N3638 2N3638A lN5059 1,000 mfd., :20%, 3 vDC 25 mfdi, l0 to +50%, 3 vDC47 pfd., :l0%, 500 vDC 0.033 mdf., 110%, 250 vDC g 1. An energizingsystem for an electrical motor in which electrical energy is passed tothe motor through a power circuit including at least one semiconductorpower switch, comprising:

a control amplifier having an input connection, and having an outputconnection connected to regulate operation of the semiconductor powerswitch and thus regulate energization of said motor,

adjustable reference means, connected to provide a speed referencesignal which varies as said reference means is adjusted, and

a linear acceleration stage,'coupled between said adjustable referencemeans and the control amplifier input connection, including anoperational amplifier connected to provide a switching signal as theadjustable reference means is varied to signal a, speed increase, acharging circuit including a capacitor coupled to the operationalamplifier to charge and provide a smooth speed change signal forapplication to the control amplifier input connection, and

afirst semiconductor switch, connected to disable the charging circuituntil the switching signal is provided by i the operational amplifier,and to rapidly discharge the capacitor as the adjustable reference meansis varied to signal a speed decrease.

2. A motor energizing system as claimed in claim 1 in which a supplyvoltage is applied to said charging circuit, which supply voltage islarger by at least an order of magnitude than the voltage developedacross said capacitor to represent the smooth speed change signal, toinsure that only the initial portion of the capacitor-charging waveformis applied to the control amplifier and provide a linear ramp outputsignal in response to a step function input signal received by thelinear acceleration stage.

3. A motor energizing system as claimed in claim 1 in which saidoperational amplifier includes a pair of input terminals and an outputterminal, circuit means completing a negative feedback between theoutput terminal and one of the input terminals, and means for applyingthe speed reference signal to the other input terminal, such that theramp increase in the smooth speed change signal is terminated bycomparing the smooth speed change signal with the speed reference signalover the negative feedback path.

4. A motor energizing system as claimed in claim 1 in which said linearacceleration stage includes a second semiconductor switch, coupledbetween said capacitor and the output conductor for applying the outputsignal from the linear acceleration stage to the control amplifier.

5. A motor energizing system as claimed in claim 4, in which both saidfirst and second transistors have base, emitter and collector elements,the base of the first transistor being coupled to the output side of theoperational amplifier, the

emitter of the first transistor being coupled to a reference conductor,the collector of said first transistor being coupled to the commonconnection between said capacitor and at least one resistance componentin the charging circuit, such that the emitter-collector path of thefirst transistor is coupled directly in parallel with said capacitor,the base of said second transistor being coupled to the collector ofsaid first transistor and to one plate of said capacitor, the collectorof said second transistor being connected to receive an energizingpotential, and the emitter of the second transistor being coupled overthe output conductor to the control amplifier.

6. A motor energizing system as claimed in claim 5 and furthercomprising a series circuit, including a first resistor and a diode,coupled between said output conductor and said reference conductor,means including a second resistor coupled to the common connectionbetween said first resistor and diode in the series circuit, forapplying a potential to said series circuit to complete the energizingpath for said second transistor.

7. An energizing system for a DC electrical motor in which electricalenergy is passed to the motor through a power circuit including aplurality of SCRs regulated by a corresponding pluralit of firingcircuits, comprising:

a contro amplifier stage having an input circuit, and having an outputcircuit connected to regulate operation of the firing circuits and thusregulate energization of the motor,

a potentiometer, connected to provide a speed reference signal whichvaries as the setting of said potentiometer is adjusted, and

a linear acceleration stage, including an operational amplifier having afirst input terminal, a second input terminal coupled to saidpotentiometer, and an output terminal for providing a switching signalas the potentiometer is adjusted to signal a speed increase, a chargingcircuit including a resistor and a capacitor coupled in series betweenan energizing connection and a reference conductor such that charging ofthe capacitor provides a smooth speed change signal for application overan output conductor to the control amplifier stage, and a transistor,having a base coupled to the control amplifier output terminal, andhaving its collector and emitter coupled to opposite plates of saidcapacitor, said transistor being normally biased for conduction todisable the charging circuit until the switching signal is provided bythe operational amplifier to turn off the transistor and allow thecapacitor to charge, and which transistor is rapidly gated on when themotor speed reaches the desired speed, to rapidly discharge thecapacitor.

8. A motor energizing system as claimed in claim 7 in which a resistoris coupled between the first input terminal of the operational amplifierand the capacitor to complete a negative feedback path, such that theramp increase in the smooth speed change signal developed by charging ofthe capacitor is terminated by comparing the smooth speed change signalwith the speed reference signal over the negative feedback path.

1. An energizing system for an electrical motor in which electricalenergy is passed to the motor through a power circuit including at leastone semiconductor power switch, comprising: a control amplifier havingan input connection, and having an output connection connected toregulate operation of the semiconductor power switch and thus regulateenergization of said motor, adjustable reference means, connected toprovide a speed reference signal which varies as said reference means isadjusted, and a linear acceleration stage, coupled between saidadjustable reference means and the control amplifier input connection,including an operational amplifier connected to provide a switchingsignal as the adjustable reference means is varied to signal a speedincrease, a charging circuit including a capacitor coupled to theoperational amplifier to charge and provide a smooth speed change signalfor application to the control amplifier input connection, and a firstsemiconductor switch, connected to disable the charging circuit untilthe switching signal is provided by the operational amplifier, and torapidly discharge the capacitor as the adjustable reference means isvaried to signal a speed decrease.
 2. A motor energizing system asclaimed in claim 1 in which a supply voltage is applied to said chargingcircuit, which supply voltage is larger by at least an order ofmagnitude than the voltage developed across said capacitor to representthe smooth speed change signal, to insure that only the initial portionof the capacitor-charging waveform is applied to the control amplifierand provide a linear ramp output signal in response to a step functioninput signal received by the linear acceleration stage.
 3. A motorenergizing system as claimed in claim 1 in which said operationalamplifier includes a pair of input terminals and an output terminal,circuit means completing a negative feedback between the output terminaland one of the input terminals, and means for applying the speedreference signal to the other input terminal, such that the rampincrease in the smooTh speed change signal is terminated by comparingthe smooth speed change signal with the speed reference signal over thenegative feedback path.
 4. A motor energizing system as claimed in claim1 in which said linear acceleration stage includes a secondsemiconductor switch, coupled between said capacitor and the outputconductor for applying the output signal from the linear accelerationstage to the control amplifier.
 5. A motor energizing system as claimedin claim 4, in which both said first and second transistors have base,emitter and collector elements, the base of the first transistor beingcoupled to the output side of the operational amplifier, the emitter ofthe first transistor being coupled to a reference conductor, thecollector of said first transistor being coupled to the commonconnection between said capacitor and at least one resistance componentin the charging circuit, such that the emitter-collector path of thefirst transistor is coupled directly in parallel with said capacitor,the base of said second transistor being coupled to the collector ofsaid first transistor and to one plate of said capacitor, the collectorof said second transistor being connected to receive an energizingpotential, and the emitter of the second transistor being coupled overthe output conductor to the control amplifier.
 6. A motor energizingsystem as claimed in claim 5 and further comprising a series circuit,including a first resistor and a diode, coupled between said outputconductor and said reference conductor, means including a secondresistor coupled to the common connection between said first resistorand diode in the series circuit, for applying a potential to said seriescircuit to complete the energizing path for said second transistor. 7.An energizing system for a DC electrical motor in which electricalenergy is passed to the motor through a power circuit including aplurality of SCR''s regulated by a corresponding plurality of firingcircuits, comprising: a control amplifier stage having an input circuit,and having an output circuit connected to regulate operation of thefiring circuits and thus regulate energization of the motor, apotentiometer, connected to provide a speed reference signal whichvaries as the setting of said potentiometer is adjusted, and a linearacceleration stage, including an operational amplifier having a firstinput terminal, a second input terminal coupled to said potentiometer,and an output terminal for providing a switching signal as thepotentiometer is adjusted to signal a speed increase, a charging circuitincluding a resistor and a capacitor coupled in series between anenergizing connection and a reference conductor such that charging ofthe capacitor provides a smooth speed change signal for application overan output conductor to the control amplifier stage, and a transistor,having a base coupled to the control amplifier output terminal, andhaving its collector and emitter coupled to opposite plates of saidcapacitor, said transistor being normally biased for conduction todisable the charging circuit until the switching signal is provided bythe operational amplifier to turn off the transistor and allow thecapacitor to charge, and which transistor is rapidly gated on when themotor speed reaches the desired speed, to rapidly discharge thecapacitor.
 8. A motor energizing system as claimed in claim 7 in which aresistor is coupled between the first input terminal of the operationalamplifier and the capacitor to complete a negative feedback path, suchthat the ramp increase in the smooth speed change signal developed bycharging of the capacitor is terminated by comparing the smooth speedchange signal with the speed reference signal over the negative feedbackpath.